Most conventional subpixelated displays utilize three emitter colors, providing a color gamut that includes the inside of a triangle when charted on the 1931 CIE Color Chart, an example of which is shown in FIG. 11. These colors are typically, substantially, red 1104, green 1106, and blue 1102. The luminance of these color emitters are typically unequal. For several reasons, some displays are constructed with a fourth color emitter. Prior art four color displays usually use white as the fourth color. This is typically done to increase the brightness of the display, as the colors are usually created using a color filter. The white is created by removing a color filter; and the light of the backlight which, being white already, is allowed to pass to the observer unobstructed. The four colors collectively are grouped into a pixel that may show any color within the triangle defined by the saturated colors, with the added ability to show lower saturation colors at a higher brightness by the addition of the appropriate amount of white.
For displays that are to be driven using a technique known in the art as Subpixel Rendering (SPR), an example of which is disclosed the '058 application, the choice of a non-filtered white subpixel creates a serious problem. Subpixel rendering depends on the ability to shift the apparent center of luminance by varying the brightness of the subpixels. This may work well when each of the colors has the same perceptual brightness. As was disclosed in co-pending and commonly assigned U.S. patent application Ser. No. 09/916,232, US Patent Publication No. 2003/0015110 (“the '110 application), entitled “Arrangement of Color Pixels for Full Color Imaging Devices with Simplified Addressing” to Elliott and herein incorporated by reference, the blue subpixels are perceived as substantially darker than the red and green, thus do not significantly contribute to the perception of increased resolution with subpixel rendering, leaving the task to the red and green subpixels. With the addition of an unfiltered white, the white subpixel, being significantly brighter than both the red and green subpixels, the red and green lose much of their effectiveness in subpixel rendering.
In FIG. 1, a prior art arrangement of four colors, sometimes called the Quad Arrangement, similar to the earlier Bayer pattern, but with one of the green subpixels replaced with a white, the repeat cell 112 consists of four subpixels, each of a different color, often red 104, green 106, blue 102, and white 108. The display is typically addressed using “whole pixel rendering” wherein the repeat cell is defined as the location of luminance information, without regard to the locations of the colored subpixels within. The colors typically have chromaticity coordinates such as those shown in FIG. 11; red 1104, green 1106, blue 1102, and white 1108. The white subpixel of this arrangement may typically be formed by removing the filter from the light path of a monochromatic LCD modulation pixel. This unfiltered white thus has significantly higher luminance than the other subpixels, which is typically the goal of the display designer.
When subpixel rendering is attempted on a four color system that has an unfiltered white, the subpixel rendering performance is substantially impaired due to the significantly higher luminance of the white subpixel. In an ideal display (of three or more color subpixel arrangement), the luminance of each of the subpixels would be equal, such that for low saturation image rendering, each subpixel has the same luminance weight. However, the human eye does not see each wavelength of light as equally bright. To the human eye, the ends of the spectrum are seen as darker than the middle. That is to say that a given energy intensity of a green wavelength is perceived to be brighter than that same energy intensity of either red or blue. Further, due to the fact that the short wavelength sensitive cones of the human eye, the “S-cones”, those giving rise to the sensation of ‘blue’, do not feed the Human Vision System's luminance channel. As a result, blue colors appear even darker.